y subterranean heat. But this is the
description of our purest fossil coals, which burn in giving the
greatest quantity of heat, and leave the smallest quantity of ashes.

In order to form another regular species of coal, let us suppose that,
along with the bituminous substance now considered, there shall be
floating in the water of the ocean a subtile earthy substance, and that
these two different substances shall subside together in an uniform
manner, to produce a stratum which shall be covered with immense weight,
compressed, condensed, and consolidated as before, we should thus have
produced a most homogeneous or uniform body to appearance, but not so
in reality. The mixture of heterogeneous matter, in this case, is too
minute to be discovered simply by inspection; it must require deep
reflection upon the subject, with the help of chemical analysis,
to understand the constitution of this body, and judge of all the
circumstances or particulars in which it differs from the former. It is
worth while to examine this subject with some attention, as it will give
the most instructive view of the composition of bituminous strata, both
those which are not considered as coal, and also the different species
of that mineral body.

In the first place then, if the mixture of those two different
substances had been sufficiently perfect, and the precipitation uniform,
the solid body of coal resulting from this mixture, would not only
appear homogeneous, but might break equally or regularly in all
directions; but the fracture of this coal must visibly differ from the
former, so far as the fracture of this heterogeneous coal cannot have
the polished surface of the pure bituminous body; for, the earthy matter
that is interposed among the bituminous particles must affect the
fracture in preventing its surface from being perfectly smooth. This
imperfect plane of the fracture may be improved by polishing; in which
case the body might be sufficiently smooth to have an agreeable polish;
but it cannot have a perfect polish like a homogeneous body, or appear
with that glassy surface which is naturally in the fracture of the pure
bituminous coal.

But this is also a perfect description of that species of coal which is
called in England Kennel coal, and in Scotland Parrot coal. It is so
uniform in its substance that it is capable of being formed on the
turning loom; and it receives a certain degree of polish, resembling
bodies of jet.

Th